Sound impingement onto the diaphragm occurs via the sound opening in the substrate and/or via the through openings in the counterelement. The diaphragm deflections resulting therefrom perpendicular to the layer planes are sensed, as changes in capacitances, with the aid of the capacitor assemblage.
The diaphragm structure reacts, however, not only to acoustic pressure but also to fluctuations in ambient pressure and to air-flow-related low-frequency pressure fluctuations such as those caused, for example, by wind. Spurious influences of this kind on the microphone signal can be reduced by a slow pressure equalization between the two sides of the diaphragm. The speed at which such a pressure equalization occurs depends substantially on the flow resistance of the corresponding flow paths. The lower the flow resistance, the more quickly a pressure equalization between the diaphragm front side and diaphragm back side takes place, and the less influence atmospheric pressure fluctuations and air flows have on the microphone signal. As the flow resistance decreases, however, so too does the microphone's sensitivity to low-frequency acoustic signals, referred to as the “roll-off” at low frequencies.
U.S. Published Patent Appln. No. 2012/0033831 describes a microphone component having variable roll-off behavior. The known microphone component encompasses an acoustically active diaphragm that functions as a movable electrode of a capacitor assemblage for signal sensing. Leakage openings for pressure equalization between the diaphragm front side and diaphragm back side are embodied in the diaphragm. The known microphone component furthermore encompasses a counterelement constituting a carrier of a stationary electrode off the capacitor assemblage. The counterelement is disposed at a distance from the diaphragm and has through openings, so that it is acoustically permeable. Because of the very short distance between the diaphragm and counterelement, in the case of the known microphone component the flow resistance between the front and back sides of the diaphragm depends substantially on the offset between the through openings of the counterelement and the leakage openings of the diaphragm, and can accordingly be varied in controlled fashion by a parallel displacement between the diaphragm and counterelement. This relative motion is produced with the aid of a drivable actuator arrangement, for example capacitively or piezoelectrically.
Although the roll-off behavior of the known microphone component can in this manner be dynamically adapted to the ambient situation, the overall sensitivity of the known microphone component is nevertheless very limited. This is attributable to the very short distance d between the diaphragm and counterelement.
The mechanical sensitivity of the diaphragm of a microphone component can be appreciably increased by application of a bias voltage. The closer this bias voltage Ubias is to the so-called “pull-in” voltage Upull-in (i.e. the voltage at which the return force of the diaphragm is overcome and the diaphragm is pulled against the counterelement), the greater the acoustic sensitivity of the diaphragm. The pull-in voltage Upull-in rises with the distance between the diaphragm and counterelement, specifically as a power of 3/2. The sensitivity of a microphone component correspondingly also rises with the distance d, specifically as a power of 1/2, when the diaphragm is acted upon by a bias voltage Ubias close to the pull-in voltage Upull-in.
With the known microphone component, however, the distance d between the diaphragm and counterelement cannot be increased arbitrarily if the roll-off behavior is to be varied by a parallel displacement between the diaphragm and counterelement. The greater the distance d, or gap, between the diaphragm and counterelement, the lower the flow resistance in the gap becomes. The influence exerted on the roll-off behavior of the known microphone component by the offset between the through openings in the counterelement and the leakage openings in the diaphragm thus also becomes less as the distance d increases. Especially when the distance d between the diaphragm and counterelement is on the order of the diameter of the through openings in the counterelement, the flow resistance in the gap becomes so low that the alignment of the through openings in the counterelement and the leakage openings in the diaphragm has practically no further influence on the roll-off behavior of the known microphone component.